Alignment layers for aligning liquid crystals are important for keeping the order of alignment of liquid crystals and realizing optical characteristics based on refractive index anisotropy of liquid crystal molecules, and are essential parts of liquid crystal display elements. Alignment of liquid crystals significantly affects display characteristics of liquid crystal display elements and thus various methods for aligning liquid crystals have been investigated. In particular, a liquid crystal display apparatus that uses a vertical-alignment-type liquid crystal layer (this apparatus is also called a VA-mode liquid crystal display apparatus) has excellent display characteristics and has been widely used in displays instead of liquid crystal display apparatuses that use horizontal-alignment-type liquid crystal layers. However, VA-mode liquid crystal display apparatuses do not necessarily have sufficient viewing angle characteristics compared to emission-type display elements and various techniques have been investigated to improve the viewing angle characteristics.
A multi-domain vertical-alignment technology (MVA) has become prevalent in improving the viewing angle characteristics of VA-mode liquid crystal display apparatuses. The MVA technology employs a domain-dividing structure to create a plurality of liquid crystal domains in one pixel. In MVA technology, controlling the inclination of liquid crystal molecules is critical in creating a domain-dividing structure and the inclination has been controlled by forming slits (openings) or ribs (projections) in electrodes. However, unlike in a conventional TN mode where the pretilt direction has been regulated by alignment films, when slits and ribs, which are linear, are used, the force of regulating alignment of liquid crystals becomes uneven within a pixel and thus a distribution occurs in terms of response speed. Another problem is that regions where slits and ribs are formed exhibit decreased optical transmittance, resulting in a decrease in display luminance.
Another technique for controlling the inclination is a polymer sustained alignment (PSA) technique in which a photo- or thermo-polymerizable monomer added to liquid crystals is polymerized by voltage application while tilting liquid crystal molecules so that the liquid crystal molecules memorize the tilt direction (refer to PTL 1). This technique can overcome the problem of response speed distribution and a decrease in optical transmittance that has occurred in the slits-and-ribs technique. However, this technique faces other problems such as changes in characteristics caused by addition of monomer to the liquid crystal material, difficulty of controlling the process, adverse effects of remaining monomers, etc.
In order to avoid these problems, it is preferable even for VA-mode liquid crystal display apparatuses to form a domain-dividing structure by controlling the inclination by using alignment films. One way to apply force for controlling the inclination to a vertical alignment film is a rubbing technique. However, it is difficult to form a precise domain-dividing structure by a rubbing technique and problems such as static electricity caused by friction and occurrence of impurity components arise.
A photoalignment technique is another technique for controlling the inclination by using an alignment film other than the rubbing process (refer to PTL 2). In a photoalignment technique, a more precise domain-dividing structure can be easily fabricated by changing the light irradiation pattern and occurrence of static electricity and impurities is less compared to a rubbing process since the alignment film can be processed without requiring contact. However, although it is widely known that the conventional photoalignment technique is used in controlling horizontal alignment, in order to control the vertical alignment which has a completely different direction of alignment from the horizontal alignment, a vertical alignment layer composition that has a structure completely different from that of a conventional photoalignment film for horizontal alignment must be used. This composition is required to exhibit an ability to control the inclination of liquid crystals at a small optical exposure dose and various characteristics, such as reliability, that make the composition applicable to active matrix driving. Materials for forming liquid crystal vertical alignment layers satisfying these requirements have not been known heretofore.
A photoalignment layer composition that has an azo-containing skeleton as a moiety that is photochemically isomerizable but not photochemically crosslinked and a cinnamic acid skeleton as a moiety that is photochemically crosslinkable has already been disclosed (refer to PTL 3, 4, and 5). However, in these literatures, the composition is used to align liquid crystal molecules horizontally with respect to a substrate and it is impossible to use the photoalignment layer composition having these skeletons in a vertical alignment layer having a completely different alignment. Moreover, since the literatures are silent as to specific means used in vertical alignment, development of polymers for use in liquid crystal alignment layers for vertical alignment has been desired.